blowers for air knives

Blowers for air knives?

Actually, it's a very good idea

Industrial air knives direct air flow toward a product and use the air’s velocity for product drying, cleaning, coating and cooling in many manufacturing and packaging plants. In their most simplistic form, holes are drilled into pipes. Involutes or air amplifier designs are used in more advanced systems. In either case, these applications require high air velocities but not high pressure.

While compressors are commonly used for air knife process applications, they are not the most energy efficient means when compared with rotary lobe blowers. A significant amount of energy is required to compress air to pressures of 80-110 psig before it is stored, dried and filtered. However, a blower is only pressurizing the air to 3-6 psig – mainly to overcome friction losses in the supply piping and through the air knife orifice – and this air does not require storage or further drying because of the low compression ratios.

Substantial energy savings can be achieved by replacing air knife processes currently using compressed air with blower-supplied air. For a given process, the flow rate will be constant, though a centralized low pressure air system may need to be designed to produce varying flows to accommodate production lines going on and off line.

Frequently asked questions (FAQs) regarding using blowers for air knives

Q: Don’t I need pressure to remove water or particles from my product?
A: Pressure in a tank is static energy. What removes water or particles from a product is kinetic energy. Once pressure leaves the pipe, it rapidly dissipates into the atmosphere. The dissipation rate gives you velocity. If your part is in range of the expanding air, the energy can be used to remove water or material from a part. A blower generates high volumes of air flow at low pressures. When it exits the pipe or nozzle, the same condition occurs, but the velocity already exists in the air flow which can be generated at a lower cost.
Q: Aren’t blowers loud?
A: Blowers can be noisy, but with properly designed air knives and a well-designed blower, noise levels in the plant can be reduced. Furthermore, a positive displacement blower can be located away from the product floor in a compressor or utility room. This can drastically reduce plant noise and free up plant floor space.
Q: When do I use blowers and when do I use compressed air?
A: This will be a function of current compressed air usage and how much low pressure air you require. If the low pressure usage is very small, it may be difficult to justify the investment, but if the low pressure demand is drawing away from the effectiveness of the compressed air system, a low pressure investment can pay for itself in just a few months.

Consider blowers for knife applications including:

  • Food Packaging:
    • Drying trays, cans and bottles
    • Removing over fill and spillage
    • Cooling hotfill packages and shrink wrapped products
    • Spraying food dyes
  • Bottling Plants
    • Can and bottle drying
  • Metal Forming
    • Drying or lubricant removal
    • Descaling and cleaning
    • Cooling for cold rolling, coating, galvanizing, plating and roll forming
  • Foundries and Casting Plants
    • Cleaning sand filled molds
    • Cooling products

Blowoff device comparison: 2 x12" Air knives for water stripping

Blowoff device type Operating pressure (psig) Air consumption (cfm) Power (hp) Maintenance costs Annual electrical cost*
flat nozzles 60-80 psig 257 cfm 60 hp same as compressor $9,742.07
drilled pipe 60-80 psig 175 cfm 40 hp same as compressor $6,387.68
venturi air knife 60-80 psig 72 cfm 20 hp same as compressor $3,280.16
low pressure air knife 2-4 psig 345 cfm 15 hp same as blower $1,165.32

* Assumes 110 psig with 30% idle power. Duty cycle determined by required cfm/unit cfm. kW calculation assumes 0.83 power factor, 8.3 cents/kWh, 40 hours/week, 52 weeks/year.

It is important to keep in mind that the amount of savings really depends on your starting point. The more rudimentary your initial arrangement, the greater your savings will be. We have added a table to the blog entry so you can see a few conclusions I was able to draw. Based on x2 12-inch air knives, I came up with 65-85% savings for switching from a compressor to a blower. As stated before, the type of device selected has an effect on savings potential.

Regarding the cost to operate a blower—comparing flow for flow, blowers are actually much cheaper than a compressor because the pressures are lower and as a result, they use less energy. For regular maintenance we are talking about quarts of oil in a blower whereas compressors we looking at gallons of oil with more involved regular maintenance. When both of these factors are considered, blowers are generally less expensive to operate.

Additional resources: